Polycarbonate-block copolymer blend with improved solvent and impact resistance

ABSTRACT

A modified polycarbonate composition comprising a blend of a bisphenol A-based polycarbonate and from greater than about 5% and less than about 15% by weight of a high molecular weight hydrogenated styrene-butadiene-styrene triblock copolymer has dramatically improved environmental stress crack resistance as well as improved impact resistance properties.

This is a continuation of U.S. patent application Ser. No. 655,241,filed Feb. 12, 1991, now abandoned, which is a continuation of Ser. No.232,256, filed Aug. 15, 1988, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates generally to modified polycarbonatecompositions with improved impact and environmental stress crackresistance properties. More particularly, the present invention relatesto blends of aromatic polycarbonates and high molecular weighthydrogenated styrene-butadiene-styrene triblock copolymers.

Aromatic polycarbonates are well known commercially available materialshaving a variety of applications in the plastics art. Generallyspeaking, these resins offer high resistance to attack by mineral acids,have high tensile strength and high impact strength, except in thicksections, good thermal resistance and a dimensional stability farsurpassing that of most other thermoplastic materials.

The use of aromatic polycarbonate resins in certain applications islimited, however, because they have a high viscosity in the melt, makingmolding of complex large and especially formed parts difficult. Theyalso exhibit brittleness under sharp impact conditions in thick sectionand, regardless of thickness, when small amounts of reinforcements suchas glass, or pigments such as titanium dioxide, are added forconventional purposes. In addition, polycarbonate resins exhibit severeenvironmental stress cracking. The term "environmental stress cracking"refers to the type of premature failure under stress which is hastenedby the presence of organic solvents, e.g., acetone, heptane and toluene,when such solvents are in contact with stressed articles fabricated fromaromatic polycarbonate resins. Such contact may occur, for example, whensolvents are used to clean or degrease stressed parts fabricated frompolycarbonates, or when such parts are used around gasoline engines inautomotive and recreational applications.

As a result, polycarbonate polymers have been modified by or blendedwith additional polymers to achieve materials with the desiredcombination of properties. For example, in U.S. Pat. No. 4,088,711polycarbonates are combined with block copolymers of a monoalkenyl arenepolymer and a completely hydrogenated conjugated diene to form acontinuous interlocking network, with the block copolymer acting as amechanical structural stabilizer. The compositions described in U.S.Pat. Nos. 4,537,930, 4,267,096 and 4,122,131 are blends ofpolycarbonates and small amounts of vinyl aromatic and olefin elastomercopolymers having improved environmental stress crack resistance. U.S.Pat. No. 4,628,072 describes the improvement of physical properties suchas adhesion, impact resistance, weatherability and heat resistance bythe addition of block copolymers of a monovinyl substituted aromatichydrocarbon polymer and unsaturated olefin compound polymer to variousthermoplastic polymers. See also U.S. Pat. No. 4,579,903 directed tocopolyester carbonate resin compositions incorporating copolymers ofvinyl aromatic compounds and olefinic elastomers which provide improvedsolvent resistance and impact properties.

The present invention is directed toward novel polycarbonatecompositions having markedly improved environmental stress crackresistance as well as improved impact resistance properties.

SUMMARY OF THE INVENTION

The present invention comprises a blend of an aromatic polycarbonatepolymer and a block copolymer comprising an alkenyl arene polymer and ahydrogenated diene elastomer. Preferably the aromatic polycarbonate is abiphenol-based polycarbonate such as bisphenol A, and the blockcopolymer is a hydrogenated styrene-butadiene-styrene triblock copolymerof a molecular weight greater than about 130,000 and present in amountsgreater than about 5% to less than about 15% of the composition byweight. All percentages given herein are by weight unless otherwiseindicated. Compositions of the present invention exhibit markedlyincreased environmental stress crack resistance and improvements inimpact resistance properties.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

The present invention comprises a blend of an aromatic polycarbonatepolymer and a block copolymer comprising an alkenyl arene polymer and ahydrogenated diene elastomer. The block copolymer is of relatively highmolecular weight and is present in an amount effective to improve theenvironmental stress crack resistance of the polycarbonate blend. Theamount of copolymer present in the blend is preferably such thatsubstantial improvement of environmental stress crack resistancerelative to unmodified polycarbonate is obtained. By "substantialimprovement" is meant at least about a 100% increase in the time to failat a constant stress of 1500 psi in 75% by volume isooctane and 25% byvolume toluene relative to unmodified polycarbonate.

Aromatic polycarbonates suitable for compositions of the presentinvention include, for example, polymers derived from diphenols such asbisphenol A, 1,1(4 hydroxyphenol)ketone, bis-(4-hydroxyphenyl)methane,1,1-bis-(hydroxyphenyl)-ethane, phenolphthalein, and 1,1bis(hydroxyphenol) sulfone; and aromatic polycarbonates with alkyl orhalogen substituents on the phenyl ring. Preferably, the polycarbonatecomponent of a blend of the present invention is a bisphenol A-basedpolycarbonate with a melt flow of from about 3 to about 80 g/10 min runat 300° C. and 3.8 kg wt (ASTM) condition O, and more preferably with amelt flow of from about 4.6 to about 15 g/10 min condition O.

Suitable block copolymers for blends of the present invention arealkenyl arene-hydrogenated diene triblock copolymer of a relatively highmolecular weight. Suitable structural arrangements of block copolymersuseful in compositions of the present invention include linear triblockcopolymers or, alternatively, for example, star block copolymers. Morepreferred are alkenyl arene-diene-alkenyl arene linear triblockcopolymers. Most preferred are hydrogenated styrene-butadiene-styrenelinear triblock elastomers. Block copolymers suitable for compositionsof the present invention are also those of a relatively high molecularweight. Suitable copolymers include those of a molecular weight ofgreater than about 70,000, and more preferably, greater than about130,000. Most preferred are block copolymers of a molecular weight ofabout 175,000.

Suitable compositions ranges of the block copolymer in a blend of thepresent invention include greater than from about 5% block copolymer,more preferably from greater than about 5% to less than about 15%, andstill more preferably from about 8% to about 12% block copolymer. Mostpreferably, the block copolymer comprises about 10% of a blend of thepresent invention. Although the block copolymer must be present in anamount to impart improved environmental stress crack resistanceproperties to the blend, too high an amount of block copolymer mayadversely affect the molding properties of the blend. By "adverselyaffects" is meant that the blend shows gross nonhomogeneity uponmolding, giving a very poor surface and a layered structure that showssome poor physical properties. Thus, although preferred ranges aregiven, they should not be construed as limiting and should be selectedto maximize the environmental stress cracking resistance withoutadversely affecting molding properties and fabrication. It should alsobe appreciated that, as more fully described below in Specific ExamplesIX and X, at the preferred concentrations the block copolymer does notform an interconnecting network in the blend.

Compositions of the present invention comprising an aromaticpolycarbonate and block copolymer can also include additional resins,preferably in amounts of less than about 50% of the final blend.Suitable additional resins for compositions of the present inventioninclude engineering resins such as acrylonitrile-butadiene-styrenecopolymer (ABS), polybutylene terephthalate (PBT), polyethyleneterephthalate (PET), nylon and polyacetal resins.

The components of the subject composition can be blended by anytechnique which effects intimate intermixing of components withoutsignificant mechanical or thermal degradation of the polymer components.For example, the components can be dissolved or dispersed in acompatible diluent, blended together to produce a homogenous dispersionor solution and the diluent removed.

One particularly convenient method for preparing blends of the presentinvention is to first dry blend particulates of each respectivecomponent. This dry blend is directly fed into a heat fabricatingapparatus such as a screw extruder or a reciprocating screw injectionmolding machine with sufficient mixing. While the particular manner ofmixing these components in heat plasticized form is not critical,sufficient mixing should be employed to ensure a uniform distribution ofeach of the components throughout the resulting blend. In addition tothe foregoing mixing procedures, other conventional mixing proceduresmay be employed including hot roll milling, kneading and the like. Thepreferred method of blending the polymer components of the presentinvention is by extrusion at a temperature and shear rate which willeffect intimate mixing without significant polymer degradation.

Any of the various types of extrusion devices capable of bringingpolymeric components into the melted state and providing a continuous orintermittent flow of the composition through the die may be employed toprepare a composition of the invention. Such devices can include singlescrew, double screw, or multiple screw extruders having either aplanetary screw or plate or both for transformation of the mixtures intofinished or semifinished products.

A preferred composition of the present invention comprises a blend of abisphenol A-based polycarbonate and a hydrogenatedstyrene-butadiene-styrene linear block copolymer. The polycarbonate isof the general structure ##STR1## where n is preferably selected toprovide the polycarbonate with a weight average molecular weight ofabout 31,000. The block copolymer is preferably of a molecular weight ofabout 175,000, where the styrene unit molecular weights are about 25,000and the butadiene unit molecular weights are about 125,000.

Suitable amounts of styrene-butadiene-styrene block copolymer inpreferred compositions of the invention includes greater than about 5%block copolymer, more preferably from greater than about 5% to less thanabout 15% block copolymer, and still more preferably from about 8% to12% block polymer. Most preferred is a blend of about 10% hydrogenatedstyrene-butadiene-styrene triblock copolymer and about 90% bisphenol Apolycarbonate.

Compositions of the present invention exhibit a dramatic increase inenvironmental stress crack resistance toward organic solvents andimproved impact resistance. Such improved solvent and impact resistanceis documented by the data summarized in the specific examples whichfollow.

SPECIFIC EXAMPLES Example I

A polymer composition of 85 parts polycarbonate with 4.6 melt flow rateand 15 parts of a hydrogenated styrene-butadiene-styrene triblockpolymer of a molecular weight of approximately 175,000, commerciallyavailable from Shell Chemical Company as Kraton® G1651, was compoundedon a Werner & Pfleiderer ZSK30 twin screw extruder through a strand die,water bath and chopper. These granules were then injection molded on aNewbury 30-ton injection molder.

Prior to compounding, the polycarbonate was dried at least four hours at121° C. and Kraton® G1651 was dried at least 16 hours at 61° C., both incirculated air ovens. The resins were then dry-blended in a Hobartplanetary mixer for at least one minute prior to loading into thecompounding hopper. The extruder was run at about 30 lb/hr rate withheating zones set from 260° to 280° C.

This composition was again dried for over four hours at 121° C. prior tomolding on the Newbury Injection Molder. The injection molding heaterswere set at 600° to 625° F. and the molding pressure was 3000 psi. Themold temperature was set at 150° F. Problems with poor surface wereencountered during the entire molding trial and the sample alsodelaminated upon tensile testing, indicating that this composition wastoo high in Kraton® G1651 for good molding properties.

Examples II through V

The following compositions were prepared by a similar procedure outlinedin Example I.

    ______________________________________                                        Example  Polycarbonate Elastomer                                              ______________________________________                                        II       90%           10% Kraton ® G1651                                 III      95%            5% Kraton ® G1651                                 IV       90%           10% Acryloid ® KM330                               V        100%           0%                                                    ______________________________________                                    

Injection molded bars of each of the above compositions were exposed toa synthetic gasoline mixture of 75% by volume isooctane and 25% byvolume toluene at various stress levels and the time observed at whichthe sample failed by rupture.

    ______________________________________                                        Time to Fail in Minutes at Designated Stress Level                            Example                                                                              3000 psi 2500 psi 2000 psi                                                                              1500 psi                                                                             1000 psi                              ______________________________________                                        II     53       95       400     566    5,333                                 III     8       11       18      84     1,000                                 IV     13       25       42      90       900                                 V       4        7       26      166    --                                    ______________________________________                                    

These data show that although 5% Kraton® G1651 shows no improvement inenvironmental stress crack resistance performance, 10% Kraton® G1651 ismuch improved compared to pure polycarbonate. The data of Example IValso indicates that another very commonly used elastomer for impacttoughening, a methyl methacrylate shell/butylacrylate core multipolymercommercially available from Rohm & Haas Company as Acryloid® KM330, doesnot improve the environmental stress crack resistance performance ofpolycarbonate.

Examples VI through VIII

Polymer samples with the following compositions were prepared and moldedin a manner similar to Example I. In this case, good surfaces wereobtained with all samples. The toughness qualities of these materialswere evaluated by measuring the Notched Izod Impact (ASTM D256) atdifferent notch sizes, temperatures, and after elevated temperatureaging. The results were as follows:

    ______________________________________                                                    Notched           131° C. Aging                                        Izod, RT 0° F.                                                                           10 mil NI                                       Example                                                                              Composition                                                                              10 mil  5 mil                                                                              10 mil 24 hr 20 hr                             ______________________________________                                        VI     5% Kraton ®                                                                          16.8    14.2 15.8   14.4  13.0                                     G1651                                                                  VII    5% Kraton ®                                                                          13.8    10.8  7.1   3.7   3.3                                      G1650.sup.a                                                            VIII   5% Kraton ®                                                                          13.2    10.8 11.8   6.0   --                                       G1652.sup.b                                                            ______________________________________                                         .sup.a Kraton ® G1650 is a hydrogenated styrenebutadiene triblock         polymer of a molecular weight of about 70,000, available commercially fro     Shell Chemical Company.                                                       .sup.b Kraton ® G1652 is a hydrogenated styrenebutadiene triblock         polymer of a molecular weight of about 50,000, available commercially fro     Shell Chemical Company.                                                  

As seen from the above data, the impact properties of the highermolecular weight Kraton® G1651 was clearly superior to the other twolower molecular weight Kraton® G materials.

Examples IX through X

Transmission electron micrographs were taken in directions perpendicularand parallel to flow in an injection molded test bar of 90%polycarbonate and 10% Kraton® G1651.

The Kraton® light colored domains do not interconnect in any directionto form and interlocking network, but instead remain discretelydispersed in the blend.

It should be appreciated that a latitude of modification, change andsubstitution is intended in the foregoing disclosure and, accordingly,it is appropriate that the appended claims be construed broadly and in amanner consistent with the spirit and scope of the invention herein.

What is claimed is:
 1. A toughened polycarbonate composition with improved environmental stress crack resistance comprising:an aromatic polycarbonate; and an alkenyl-arene-diene-alkenyl-arene block copolymer in an amount greater than 5% by weight of the composition and effective to improve the environmental stress crack resistance of the composition relative to the aromatic polycarbonate, but in an amount less than that which adversely affects the molding properties of the blend.
 2. The composition of claim 1, wherein the block copolymer is present in an amount greater than from about 5% to less than about 15% by weight of the composition.
 3. The composition of claim 1, wherein the average molecular weight of the block copolymer is greater than about 70,000.
 4. The composition of claim 1, wherein the block copolymer is a linear block copolymer of hydrogenated styrene-butadiene-styrene.
 5. The composition of claim 1, wherein the block copolymer is a star block copolymer of styrene and hydrogenated butadiene.
 6. The composition of claim 1, wherein the aromatic polycarbonate is a bisphenol A-based polycarbonate.
 7. The composition of claim 1, wherein the aromatic polycarbonate is an aromatic ester copolycarbonate.
 8. The composition of claim 1, further comprising an engineering resin present in an amount of less than about 50% by weight of the composition.
 9. The composition of claim 2, wherein the block copolymer is present in an amount of from about 8% to about 12% by weight of the composition.
 10. The composition of claim 4, wherein the molecular weight of the block copolymer is greater than about 130,000.
 11. The composition of claim 8, wherein the engineering resin is selected form the group consisting of acrylonitrile-butadiene-styrene copolymer, polybutylene terephthalate, polyethylene terephthalate, nylon, polyacetal resin and mixtures thereof.
 12. A thermoplastic blend comprising:an aromatic polycarbonate; and a hydrogenated styrene-butadiene-styrene block copolymer present in an amount of greater than about 5% to less than about 15% by weight of the blend.
 13. The blend of claim 12, wherein the block copolymer has a molecular weight of greater than about 130,000.
 14. The blend of claim 12, wherein the block copolymer is present in an amount of about 10% by weight of the blend.
 15. The blend of claim 12, wherein the polycarbonate is bisphenol A polycarbonate.
 16. The blend of claim 12, further comprising less than about 50% by weight of an engineering resin.
 17. A polycarbonate blend comprising:a bisphenol A-based polycarbonate; and a hydrogenated styrene-butadiene-styrene triblock copolymer discretely dispersed in the blend, the copolymer being of a molecular weight and present in an amount greater than 5% by weight of the composition and effective to significantly improve the environmental stress crack resistance of the blend relative to the polycarbonate.
 18. The blend of claim 17, wherein the triblock copolymer has a molecular weight of greater than about 130,000 and is present in an amount of about 8% to about 12% by weight of the blend.
 19. The blend of claim 17, further comprising less than about 50% by weight of an engineering resin selected from the group consisting of acrylonitrile-butadiene-styrene copolymer, polybutylene terephthalate, polyethylene terephthalate, nylon, polyacetal resin and mixture thereof.
 20. The blend of claim 18, wherein the triblock copolymer has a molecular weight of about 175,000 and is present in an amount of about 10% by weight of the blend. 